1. Field of the Invention
This invention relates generally to x-ray imaging systems and more specifically to security inspection systems.
2. Description of Related Art
Systems that can form an image of an item are used in a variety of applications. Many such systems construct images using information acquired from the interaction of penetrating radiation with an item under inspection. Different systems may be constructed to gather information following different types of interactions. For example, transmission systems illuminate an item with penetrating radiation, such as x-rays, and measure the radiation after it has passed through the item. By computing the attenuation of the radiation as it passed through the item, information about objects within the item can be determined and can be used to form an image of the item, including objects within it.
Other systems acquire information on an item by measuring radiation that has been scattered by the item. For example, an inspection system may illuminate a first column of an item with penetrating radiation and measure radiation emanating from the item in a second column. These columns are positioned to intersect at a region and the measured radiation represents radiation scattered from the region as a result of interaction between the incident beam and material in this region. Because the amount of scattering depends on material characteristics, the measured radiation can be used as an indication of the type of material in that region.
These systems obtain information about objects that are inside an item and therefore hidden from direct observation. This capability has led to use of imaging systems in security inspection systems. In a security inspection system, information obtained with the imaging system is analyzed to identify contraband objects within an item under inspection without the need to physically open the item.
Multiple analysis approaches are used. Some security inspection systems render information on a visual display for analysis by a human operator who is trained to recognize images of contraband items. In other systems, an image is processed by a computer that can detect a pattern of values that is characteristic of a contraband item. For example, a gun, knife or other metal object that could be used as a weapon interacts with radiation very differently than clothes or other items commonly found in a suitcase. Accordingly, an image of an item may contain a pattern of values that can be recognized as a weapon. Other contraband items, such as explosives, may similarly produce a pattern in an image that is sufficiently distinctive to allow those contraband items to be detected upon analysis of an image formed using penetrating radiation.
The ease with which contraband can be detected with security inspection system has led to widespread use of such systems in the airline travel industry. However, security inspection systems are used in other settings. Systems that can inspect cargo containers and other packages also are in use. Some of these systems are mounted on trucks and other vehicles so that they can be moved to inspect large stationary objects.